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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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MILES AND GRINNELL GLACIERS
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feet high, with the zone of thin moraine standing distinctly higher than that of barren thick moraine. The eastern edge of this zone is at essentially the same level as the clean ice of the valley glacier.
Cause for Moraine Zones. The separation of the northern portion of Miles Glacier bulb into three distinct zones calls for explanation, for the condition is distinctly not that of gradation from zone to zone, but of sharp contrast (PL CLXTV). In the table below the significant facts are contrasted.
TABLE CONTRASTING CONDITIONS IN THE THREE ZONES OF ABLATION MORAINE ON
MILES GLACIER
Zone	Moraine Cover	Slumping	Vegetation	Age of Trees
Thickest moraine Thick moraine Thin moraine	Several feet A few feet A few inches	Very little A great deal 'Rftpid T^plting	Continuous Scattered None	50 to 75 years 14 to 22    "
				
Fig. 68 is a cross-section showing the three zones graphically. The east-facing bluff at b, and the west-facing ice cliff at c, as well as the marked difference in ages of trees, are thought to be especially significant in connection with the problem as to the difference of conditions in the forested zone (a-b, PL CLXIV), the relatively-barren zone (b-c), and the absolutely-barren zone (c-d).
At c, along the contact of the zones of thick and thin moraine, an advance was in progress or was just ceasing in 1910. At the edge of the lake it was clearly still in progress, for ice blocks were sliding down from an ice cliff which was not undercut by the
3000 feet
VALLEY
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Fro. 63.   NATURAL SOALB CBOSS-SHOTION OF COPPER Rrvma VALLEY.
waves, and they lay in a heap on the beach. Back from the lake shore the snow of the winter of 1909-10 was folded up in front of the advancing ice edge. While we could not be certain that advance was still taking place all along the low cliff on the line c-c (PL CLXV), it was certainly still in progress for a quarter of a mile or so from the lake shore. The ice in the zone of thin moraine had the appearance of being thrust-faulted forward over the ice in the zone of thick moraine.
The explanation that naturally suggests itself is that an advance of Miles Glacier a few years before 1910 crevassed the ice in the zone of thin ablation so that the vegetation which had previously straggled across into this belt was destroyed, while the gradually-thinning morainic cover was partly swallowed up in the crevasses. This advance, of which there is independent photographic evidence, began after 1908 and had ap-